 /*******************************************************************************
  * Copyright (c) 2000, 2006 IBM Corporation and others.
  * All rights reserved. This program and the accompanying materials
  * are made available under the terms of the Eclipse Public License v1.0
  * which accompanies this distribution, and is available at
  * http://www.eclipse.org/legal/epl-v10.html
  *
  * Contributors:
  * IBM Corporation - initial API and implementation
  *******************************************************************************/

 package org.eclipse.ui.actions;

 import org.eclipse.jface.action.Action;
 import org.eclipse.jface.viewers.ISelection;
 import org.eclipse.jface.viewers.ISelectionChangedListener;
 import org.eclipse.jface.viewers.IStructuredSelection;
 import org.eclipse.jface.viewers.SelectionChangedEvent;
 import org.eclipse.jface.viewers.StructuredSelection;
 import org.eclipse.swt.widgets.Event;

 /**
  * The abstract superclass for actions that listen to selection change events.
  * This implementation tracks the current selection (see
  * <code>getStructuredSelection</code>) and provides a convenient place to
  * monitor selection changes that could affect the availability of the action.
  * <p>
  * Subclasses must implement the following <code>IAction</code> method:
  * <ul>
  * <li><code>run</code> - to do the action's work</li>
  * </ul>
  * </p>
  * <p>
  * Subclasses may extend the <code>updateSelection</code> method to update
  * the action determine its availability based on the current selection.
  * </p>
  * <p>
  * The object instantiating the subclass is responsible for registering
  * the instance with a selection provider. Alternatively, the object can
  * notify the subclass instance directly of a selection change using the
  * methods:
  * <ul>
  * <li><code>selectionChanged(IStructuredSelection)</code> - passing the selection</li>
  * <li><code>selectionChanged(ISelectionChangedEvent)</code> - passing the selection change event</li>
  * </ul>
  * </p>
  * @since 3.0
  */
 public abstract class BaseSelectionListenerAction extends Action implements
         ISelectionChangedListener {
     /**
      * The current selection.
      */
     private IStructuredSelection selection = new StructuredSelection();

     /**
      * Running flag: <code>true</code> iff the action is running.
      */
     private boolean running = false;

     /**
      * The deferred selection. Any selection change that occurs
      * while the action is running is held here until the run is complete.
      */
     private IStructuredSelection deferredSelection = null;

     /**
      * Creates a new action with the given text.
      *
      * @param text the string used as the text for the action,
      * or <code>null</code> if there is no text
      */
     protected BaseSelectionListenerAction(String text) {
         super(text);
     }

     /**
      * Clears any cached state associated with the selection.
      * Called when the selection changes.
      * <p>
      * The <code>BaseSelectionListenerAction</code> implementation of this method
      * does nothing. Subclasses may override.
      * </p>
      */
     protected void clearCache() {
         // do nothing
 }

     /**
      * Returns the current structured selection in the workbench, or an empty
      * selection if nothing is selected or if selection does not include
      * objects (for example, raw text).
      *
      * @return the current structured selection in the workbench
      */
     public IStructuredSelection getStructuredSelection() {
         return selection;
     }

     /**
      * Notifies this action that the given structured selection has changed.
      * <p>
      * The <code>BaseSelectionListenerAction</code> implementation of this method
      * records the given selection for future reference and calls
      * <code>updateSelection</code>, updating the enable state of this action
      * based on the outcome. Subclasses should override <code>updateSelection</code>
      * to react to selection changes.
      * </p>
      *
      * @param selection the new selection
      */
     public final void selectionChanged(IStructuredSelection selection) {
         // Ignore any incoming selection change while the action is running,
 // otherwise the action can have unpredictable results, including lost
 // data, if it operates on a different selection than what it initially
 // validated.
 // See Bug 60606 [Navigator] (data loss) Navigator deletes/moves the wrong file
 if (running) {
             deferredSelection = selection;
             return;
         }
         this.selection = selection;
         clearCache();
         setEnabled(updateSelection(selection));
     }

     /**
      * The <code>BaseSelectionListenerAction</code> implementation of this
      * <code>ISelectionChangedListener</code> method calls
      * <code>selectionChanged(IStructuredSelection)</code> assuming the selection is
      * a structured one. Subclasses should override the <code>updateSelection</code>
      * method to react to selection changes.
      */
     public final void selectionChanged(SelectionChangedEvent event) {
         ISelection selection = event.getSelection();
         if (selection instanceof IStructuredSelection) {
             selectionChanged((IStructuredSelection) selection);
         } else {
             selectionChanged(StructuredSelection.EMPTY);
         }
     }

     /**
      * Updates this action in response to the given selection.
      * <p>
      * The <code>BaseSelectionListenerAction</code> implementation of this method
      * returns <code>true</code>. Subclasses may extend to react to selection
      * changes; however, if the super method returns <code>false</code>, the
      * overriding method must also return <code>false</code>.
      * </p>
      *
      * @param selection the new selection
      * @return <code>true</code> if the action should be enabled for this selection,
      * and <code>false</code> otherwise
      */
     protected boolean updateSelection(IStructuredSelection selection) {
         return true;
     }

     /* (non-Javadoc)
      * @see org.eclipse.jface.action.Action#runWithEvent(org.eclipse.swt.widgets.Event)
      */
     public void runWithEvent(Event event) {
         // Set the running flag during the run so that selection changes are deferred.
 // See selectionChanged(IStructuredSelection) for more details.
 running = true;
         try {
             run();
         } finally {
             running = false;
             IStructuredSelection s = deferredSelection;
             deferredSelection = null;
             if (s != null) {
                 selectionChanged(s);
             }
         }
     }
 }

